The present invention relates to a fluorescent lamp control circuit for operating the lamp from a direct current source and more particularly to a dimming circuit for such a lamp. The invention further relates to a protection circuit for the lamp control which operates when the circuit is in an unloaded condition.
An invertor circuit which utilizes a magnetic force for providing an oscillating source for driving a fluorescent lamp is described in Japanese Utility Model Gazette No. 54-2598, 48-23181. However, in this device it is impossible to control the brightness of the lamp. Furthermore, in the disclosed circuit, there is no emergency mechanism for protecting the circuit against failures in the unloaded condition, where the lamp is not completely connected to the circuit, such as might occur with a bad cutting of a lamp filament, wherein an excess current may cause damage to the dimming circuit or the possible danger of a fire. Despite the importance of restricting excess currents at the unloaded condition, most lamp control circuits previously proposed do not provide a sufficient protection function, and some do not even employ a protection apparatus at all. On the other hand, it is conventional to provide means for controlling the brightness of a lamp, but the circuit architecture for such a controller is very complicated and its control range is not satisfactory, such devices generating a flickering light at low levels of illumination. Furthermore, the manufacturing cost of such prior devices is high, so that such controls have not been feasible in general.